A COLOSSAL plume of freezing air will sweep down from the Arctic and in from eastern Europe THIS WEEK engulfing Britain in sub-zero temperatures until SPRING.

Heavy snow, blizzards, freezing gales and lethal ice sheets threaten to plunge the nation into the most severe winter lockdown for six years.

Freak changes in the upper atmosphere threaten to displace the Polar Vortex – a pool of cold air which circulates the North Pole and Siberia – dragging freezing, snow-laden air across the UK.

Despite the milder, stormier weather over the next couple of days forecasters warn to brace for a dramatic change to bitter conditions.

Thermometers are tipped to hit -10C (14F) in the north by the end of next week with the south plummeting to around -3C (26.6F).

Experts warn to brace for a shock to the system after a relatively calm winter so far which brought a brief burst of snowy weather at the start of January.

They say the next blast will whip up crippling blizzards and feet-deep snowdrifts grinding the country to a standstill until March.

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UK is on alert for snow this week as the weather turns nasty

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The temperatures on Friday at 7am will plunge

James Madden, forecaster for Exacta Weather, said rain will start turning to snow over high ground in the north this weekend.

He said: “Towards the end of this week and into the weekend will see things starting to turn colder from the north of the country.

“This will initially bring the risk of rain turning to snow across parts to the north and west of the country, and even some parts as far south and the Midlands or Central England could see some wintry showers popping up in places.

“This will then pave the way for what is now likely to be a very cold weather pattern with multiple snow events throughout the remainder of February.”

The sharp change in weather will be driven by a Sudden Stratospheric Warming (SSW) which will open the floodgates to cold air from the north and east.

Sudden Stratospheric Warming is triggered by westerly winds reversing to easterly causing air in the stratosphere to spill into the Polar cap where it compresses and warms.

This in turn blocks milder air coming in from the Atlantic and draws cold air in from the Arctic regions as the jet stream drifts southwards.

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Wind chill is expected to make temperatures feel particularly cold

SSW works hand in hand with the Arctic Oscillation (AO) – the relationship between low pressure over Iceland and high pressure further south.

When this pressure difference weakens the AO enters a ‘cold’ or negative phase further helping to weaken the westerly winds and allowing cold Arctic air to sink southwards.

Mr Madden said Britain is likely to remain in this set up through the rest of February and into the start of spring.

He said: “We are now looking at a very cold weather pattern with multiple snow events throughout the remainder of February.

“This will in part be due to a significant stratospheric warming event that is currently occurring.

“It is allowing the polar vortex near the North Pole to weaken further in the coming weeks as these developments start to gradually impact our lower atmosphere.

“This will allow cold Arctic air and east to north-east winds to spill across the country and encase us in much colder than average temperatures for much of February.

“The stagnated cold air across our shores will then heighten the risk for multiple snow events, particularly from in and around February 10 to 15.

“This can also all be monitored as being on track to occur and with high confidence as to how negative the Arctic Oscillation (AO) will become within this specified period.”

The Met Office said weather models have changed over the past few days and are showing signs of cold weather returning next week.

Forecaster Emma Sharples said: “it could turn colder next week, things have changed a bit since the start of the week.

“The high pressure system over Scandinavia and Siberia will reform, blocking Atlantic weather patterns and allowing an easterly set up.

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“Over the past few days there have been signals it will turn colder again.”

The Met Office’s long-range outlook warns a largely mild start to next week will give way to bitter winds and the risk of snow.

It states: “Although temperatures will be largely mild initially, there is likely to be some colder spells, especially later, bringing the risk of wintry showers, mainly in the north, with cold and bright conditions in the east.

“From the middle of the month the most likely scenario is for unsettled weather in the west, with wet and windy spells interspersed with brighter, colder and showery interludes.

“There is a risk of gales at times, particularly in the west, where the heaviest rainfall is also most likely.

“Later in February and into March there are signs of a quieter and rather colder spell developing more widely.”

Britain’s winters have been relatively mild since the last big freeze of 2010/2011 which brought heavy snow and record low temperatures.

December turned out to be the coldest for more than 100 years with temperatures hovering around 4.8C below average.

Heavy snow grounds transport networks to a standstill while thousands of air passengers were left stranded for days after flights were grounded.

Subsequent years have been milder, wetter and windier with this winter so far following a similar trend although experts warn February will buck this trend.